Yes, you obviously were way too stressed out after committing yourself to so many cakes in one weekend. One was bound to have a problem. Your cake was great, and in my opinion the main problem was that you thought so little of your talent that you gave it for free. If I was the client, I'd certainly suck it up about a cracked crown if it meant I'd get the cake for nothing. Live and learn, that's all anyone can do. Don't feel badly, you more than compensated them.